1. Field of the Invention
This invention relates to a circuit board using an aluminum or like other metal plate as a substrate.
2. Description of the Prior Art
As the mounting density of components has become increased with the recent trend of reducing the size of various electronic equipments, circuit boards having a substrate made of a metal plate which can efficiently dissipate heat generated from these components and excellent in mechanical strength have now been used frequently. A circuit board of the above-mentioned type is constituted such that a conductive layer, for example, made of a copper foil having a predetermined pattern is layered on a metal plate, such as made of aluminum by way of a resinous layer having a good insulation property. However, upon applying bending work or the like to such a circuit board, since the resinous layer generally rigid in nature can not afford a sufficient elongation necessary for the bending, crackings or like other defects may possibly result in the resinous layer to reduce the dielectric strength in the bent portion, or disconnection may be caused to the conductive layer because the copper foil which is usually employed for the conductive layer has a poor elongation as low as about 10%. Thus, the circuit board of the foregoing structure has generally been used substantially in a flat state.
Consequently, it is impossible for an equipment of a complicated shape to effectively utilize the inner space, for instance, by enclsoing circuit boards therein while optionally bending them along the space and thereby rendering the size and the shape of the equipment compact, whereby the foregoing size-reducing demand for electronic equipment has not yet been satisfied completely.
In view of the above, some use has been made of soft resin materials for the resinous layer of the circuit board so as to overcome the foregoing troubles such as the crackings in the resinous layer or the disconnection of the conductive layer by the elongation of the resinous layer and thereby rendering the circuit board adaptible to the bending fabrication.
However, soft resin materials are generally poor in electrical insulating performance and, particularly, there have not been known any soft resin materials capable of maintaining sufficient dielectric strength after a moisture-proof test. It is thus difficult to obtain a circuit board having a predetermined dielectric strength and such a board has not been put to practical use.